Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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This is where building a prototype becomes so critical. It is easier to make adjustments on one cavity than on a 64-cavity mold. The prototype can be used to develop the automation strategy for a consistent process and cycle time. It also helps to work with mold and machine builders that have experience in a wide range of LSR molding solutions and that can provide reliable, high-quality automation. The press must be equipped with the necessary EuroMap connection to allow for automation integration. As automation becomes ever more sophisticated, it is necessary for the press to be able to talk to the robot and other automation so that control is as seamless as possible and the safeties are maintained on the press. MACHINE ISSUES To run LSR efectively, the press needs an adequately sized vacuum pump that is capable of evacuating all the mold cavities quickly. If the vacuum pump is too small, this will increase cycle time while waiting for the cavities to be evacuated. One important option on the press is provisions built into the machine controls to run a valve-gated cold deck. While it is possible to use a separate valve-gate controller, this adds another layer of equipment to the process, which is less desirable. Considering the variety of jobs a machine may be used for, new machines must be outftted with the proper number of valve-gate controls. A good rule of thumb is to start with no fewer than eight valve-gate control solenoids. Most cold decks are made to be divisible by eight. For a four- cavity mold, you can use four of the eight solenoids. In an eight- cavity mold, you can use one solenoid for one gate. This ofers the best degree of control, but limited control can be achieved by operating multiple valve gates of the same solenoid. For example, with a 16-cavity mold, you can use the bank of eight solenoids to turn on eight pairs of valve gates at a time to give you limited control. A similar approach applies to operating 32 and 64 drops with eight sole- noids. Clearly, it is desirable for the molder to have a clear idea of the parts and sizes of molds that will be run in the press so that the machine is developed originally with the optimum number of controls. The press also needs to provide proper feedback for trouble- shooting the process within the cycle parameters. The feedback mechanisms must include the capability of reading the mold heats QUESTIONS ABOUT LSR MOLDING? See the LIM & LSR Molding Zone for more articles. Molding LSR typically requires a machine capable of handling small shot sizes accurately and repeatably. That's one reason for the popularity of all-electric machines in this feld, like this Milacron-Fanuc Roboshot press in Roembke's lab. Outftting a new press with a suffcient number of valve-gate solenoids (visible here to the right of the water regulator) takes some forethought about the range of molds it will run. Most cold decks come in units of eight gates. ABOUT THE AUTHORS: Troy Smith is global sales director at Roembke Mfg. & Design, Inc., Ossian, Ind., which specializes in molds for LSR and other rubbers. He has been at Roembke for more than 32 years. (260) 622-4135, x 3130; tsmith@roembke.com; roembke.com. Andy Stirn is global electric machine product manager for Milacron LLC, Batavia, Ohio, and is involved with product development and application of systems for electric and packaging machinery. (513) 536-2000; andy_stirn@milacron.com; milacron.com/plastics. and giving a visual display of all zones and how they are working. Additional important feedback includes whether the machine is pulling adequate vacuum on the mold, as well as the injection speeds, hold times, and injection pressures from shot to shot. Adding mold- cavity sensors is an important advantage for gaining full control of the molding cycle.Molding small parts requires the machine and process to be able to control very small shot sizes accurately and repeatedly. The best machines for repeatability are ones in which all electrical motors/ drives, software, and controls are developed from one company. Customers should look for machines that can produce LSR and small parts with tight tolerances and with high levels of motion control. Several defects can signal improper venting—short shots, knit lines, fow lines. Improper mold heating can produce such defects, too. @plastechmag 57 Plastics Technology L S R I N J E C T I O N M O L D I N G

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